With the introduction of digital user terminals, such as digital telephones in digital telephone extension systems, accessory functions can be made available to users that could not be provided heretofore by analog type conventional telephones. Among such accessory functions are services such as an alphabetically and numerically arranged directory, caller identification and name entry, transmission of brief messages from one user to another, or alarm functions for schedule monitoring.
These accessory functions are note directly related to the communications exchange, and are therefore not normally supplied by the exchange itself. Instead accessory functions are most conveniently implemented by employing one or more accessory function modules which are capable of communicating with the user terminals.
Communication between the accessory function module or modules and the user terminals may be carried out in a number of ways. Such communication might be accomplished, for example, byway of separate lines from the accessory function module or modules to each user terminal. This approach would be costly, of course because of the extensive circuitry that would be required by the numerous separate lines. Another disadvantage of the use of separate lines would be that the cost of refitting existing digital telecommunications systems with accessory function modules would be prohibitively high.
Alternatively, communication of the accessory function module or modules with the user terminals might be accomplished in the same way as communication between two user terminals, namely by assignment of a pair of time slots through the central exchange. While this approach is attractive in that existing telecommunication systems could be reequipped, an additional burden is nevertheless placed on the exchange system which is unrelated to the function of the exchange system itself.
The object of the present invention, therefore, is to provide a method of communication between one or more accessory function modules and the user terminals that requires the use of only a modest amount of additional circuitry and that does not place an additional burden the exchange system.